move toEpoch to helpers.cpp, use strptime

Origin commit data
------------------
Commit: 52741fb1e4
Author: Jacek Jendrzej <overx300@gmail.com>
Date: 2014-01-04 (Sat, 04 Jan 2014)

src/system/helpers.h

@@ -47,6 +47,7 @@ std::string getFileName(std::string &file);
 std::string getFileExt(std::string &file);
 std::string getNowTimeStr(const char* format);
 std::string trim(std::string &str, const std::string &trimChars = " \n\r\t");
+time_t toEpoch(std::string &date);
 
 class CFileHelpers
 {